docs: Fix Documentation Accuracy, Dead Links, and Grammar in Docs

[rajarsheechatterjee]

• Correct API docs against actual src/types/plugin.ts and libs/*: fix FilterToValues type name, add missing PluginBase fields (customJS, customCSS, webStorageUtilized, resolveUrl), add missing SourceNovel.rating and ChapterItem.scanlator fields, fix ExcludableCheckboxGroupValue's include/exclude keys, document the NovelStatus enum
• Fill in the previously-empty "Using Cheerio" and "Custom fetching functions" sections, and add a new "Other libraries" section (isUrlAbsolute, storage variants, AES helpers)
• Write real content for quickstart.md's "Creating multi-source plugins" section instead of a bare heading, and clarify the lang-folder-name mismatch between plugins/ and public/static/src/
• Fix broken/empty markdown links, mismatched TOC anchors, a duplicate anchor id, and a malformed GitHub alert callout
• Normalize all code examples to the repo's own Prettier style (2-space, single-quote)
• Fix typos, grammar, and copy-paste errors (Desciption, it's/its, duplicated parseChapter description, SesesionStorage, run-on pre-submission checklist)
• Cross-link testing.md and website-tutorial.md; add missing testing.md entry to README's docs index; add a title to komga-plugin.md